Dwarf AGNs from Optical Variability for the Origins of Seeds (DAVOS): insights from the dark energy survey deep fields

Abstract

We present a sample of 706, z < 1.5 active galactic nuclei (AGNs) selected from optical photometric variability in three of the Dark Energy Survey (DES) deep fields (E2, C3, and X3) over an area of 4.64 deg2. We construct light curves using difference imaging aperture photometry for resolved sources and non-difference imaging PSF photometry for unresolved sources, respectively, and characterize the variability significance. Our DES light curves have a mean cadence of 7 days, a 6 year baseline, and a single-epoch imaging depth of up to g ∼ 24.5. Using spectral energy distribution (SED) fitting, we find 26 out of total 706 variable galaxies are consistent with dwarf galaxies with a reliable stellar mass estimate (M* < 109.5 M⊙; median photometric redshift of 0.9). We were able to constrain rapid characteristic variability timescales (∼ weeks) using the DES light curves in 15 dwarf AGN candidates (a subset of our variable AGN candidates) at a median photometric redshift of 0.4. This rapid variability is consistent with their low black hole masses. We confirm the low-mass AGN nature of one source with a high S/N optical spectrum. We publish our catalog, optical light curves, and supplementary data, such as X-ray properties and optical spectra, when available. We measure a variable AGN fraction versus stellar mass and compare to results from a forward model. This work demonstrates the feasibility of optical variability to identify AGNs with lower black hole masses in deep fields, which may be more ‘pristine’ analogs of supermassive black hole seeds.

Overmassive Black Holes in Dwarf Galaxies Out to z ∼ 0.9 in the VIPERS Survey

“…Using CIGALE we find that the seven BLAGN have stellar masses of log M *  10 M e and thus are consistent with lowmass galaxies. The AGN fraction is found to range from 17% to 42% with a mean value of 29%, similarly to the other few AGN dwarf galaxies found at z > 0.4 (Halevi et al 2019;Burke et al 2022).…”

“…The code performs a multicomponent spectral by combining host galaxy templates, a dust reddening map, Fe II templates, and an input line-fitting parameter list (Shen et al 2019). The code was initially designed to measure spectral properties of SDSS quasars, but it can be tuned to fit VIPERS (and any other) spectra (Burke et al 2022). The fitting is performed in the rest frame and using a χ 2 -based method.…”

“…An AGN dwarf galaxy at a similar spectroscopic redshift (z ∼ 0.56) and X-ray luminosity (L 0.5-10 keV ∼ 10 43 erg s −1 ) was also found by Halevi et al (2019;HSC-XD 52). Based on shorttimescale variability in Dark Energy Survey (DES) lightcurves, Burke et al (2022) have recently identified a dwarf galaxy at a spectroscopic redshift z = 0.8194 and classified as a "QSO" by the Sloan Digital Sky Survey (SDSS; J022305.3-042800.9). Based on the presence of broad Hβ and Mg II emission lines, the authors report a black hole mass of log M BH = 6.4-6.6 M e , slightly larger than those typically found at z  0.15 (e.g., Reines et al 2013).…”

Finding of a Population of Active Galactic Nuclei Showing a Significant Luminosity Decline in the Past ∼103–104 yr

“…It is naively expected that the stellar mass of low-metallicity AGNs would reside in relatively low stellar mass galaxies, i.e., M å < 10 10 M e , as shown in star-forming galaxies at each cosmic epoch (e.g., Tremonti et al 2004;Erb et al 2006;Lee et al 2006). In addition, some studies suggest that AGNs in low stellar mass galaxies tend to show a higher AGN variability amplitude in the optical bands (e.g., Kimura et al 2020;Burke et al 2022), while some report that mass dependence is weak (Caplar et al 2017). On the other hand, it is still not clear whether lowmetallicity AGNs reside in low stellar mass galaxies.…”

Intermediate-mass black holes: finding of episodic, large-scale and powerful jet activity in a dwarf galaxy

“…Many studies at X-ray and infrared wavelengths have been carried out to search for active galactic nucleus (AGN) in dwarf galaxies (e.g. Mezcua & Domínguez Sánchez 2020;Ferré-Mateu et al 2021;Molina et al 2021;Burke et al 2022;Salehirad et al 2022). Hunting for IMBHs can help to constrain the BH occupation fraction in dwarf Reines et al (2020) galaxies (e.g.…”